In heated-air drying, the seed is usually dried in a special bin, chamber, column, or tower, and then transferred to other buildings for storage. Stationary batch-type facilities are used mainly on farms, but may be used in custom and commercial drying. Portable, rubber-tired units are available. They can be pulled by tractor or truck to different locations.

The continuous-type drier generally a large, stationary unit is used most commonly in commercial drying. In most of them, the seeds move downward by gravity during the drying process and are discharged at the bottom of the tower, where they are elevated into the storage or processing plant. Sometimes more than one pass through the drier is required to reduce the moisture content to the desired level. The continuous unit is often called a self-contained drier because it holds the seeds during drying and has the built-in features of heater, fan, controls, and, often, elevating and unloading mechanisms.

Another drier type consists of a rotating drum, which lifts the seeds and drops them continuously through a heated air supply.

Fans used with unheated air must be designed to operate against the resistance encountered in forcing air through the seed mass. Suitable fans include the centrifugal, propeller, and axial types.

The horsepower requirements for operating seed-drying fans vary considerably. The power depends on the design and size of the fan, operating conditions, the amount of air delivered, kind and moisture of the seeds, and depth of seeds to be dried.

Only one control is essential in drying with unheated air a switch to turn the fan on and off. Automatic operation of fans can be had at low cost by means of a humidistat, which permits the fan to operate only when the relative humidity is below a desired level. A relay usually is required with the humidistat.

Operating the fan during loading of the bin will help to blow out some of the chaff and other foreign matter and may help keep the seed cool.

The fan should be operated continuously for most seed (except during long rainy periods) until the moisture is down to about 17 percent (wet basis).

Exact humidistat settings to obtain greatest efficiency in drying seeds further to safe storage moisture content cannot be stated, because they vary with the temperature and kind of seed.

Generally, for summer drying, the humidistat must be set so that the fan will operate only when the relative humidity is below 70 percent. With the air temperature around 80 , this setting is needed to obtain reasonable efficiency in drying to 13 percent (wet basis). Drying to 11-percent moisture will require additional operation of the fan when the relative humidity is below 50 percent. This also applies to fall drying with temperatures no lower than 50 to 55 .

Drying is extremely slow at temperatures below 50 , and an even lower humidistat setting is required. The fan should be operated two or three times a day, at least 15 minutes each time, during long periods of rainy weather if the moisture in the seed exceeds 15 percent. This will help the seed cool and prevent overheating.

HEATERS for driers are direct fired or indirect fired.

In the direct-fired type, the combustion gases go directly from the burner into the drying airstream and thence into the drying bin. The efficiency in the use of the fuel can be more than 90 percent.

Direct-fired oil-burning heaters are somewhat less safe than the indirect-fired type, because the oil burner may release particles of hot soot if it is not properly adjusted. Most gas-fired driers are of the direct-heat type. Because particles of hot soot do not form with gas, the danger of fire is less.

The indirect-fired type has a heat exchanger with a smokestack. It is like a house furnace in operation. The efficiency of this type usually is not more than 65 or 70 percent, as some of the heat escapes through the smokestack. Oil is the commonest fuel. Coal, coke, natural gas, and liquefied petroleum gas also are used.

Fan requirements for heated-air drying are about the same as for unheated air. The fan, when mounted in a drier, however, usually delivers less than the manufacturer's rating because of additional resistance encountered. Some allowance should be made for this added resistance in predicting the horsepower requirements for different situations and seeds.

DRIERS come in various sizes. The small farm driers burn 1 to 5 gallons of fuel oil an hour, or the equivalent in another kind of fuel. The larger units burn 10 to 15 gallons an hour. Commercial driers usually burn much more.

Farm driers generally have electric motors of 3, 5, or 7.5 horsepower and fan sizes to fit the motors. Fans also can be driven by gasoline engines or by tractors from the power takeoff or belt pulley.

The size of drier a farmer needs will depend mostly on how much seed he must dry in an hour, the kind of seed, the first and final content of moisture, and the season.

A burner capacity of about 1 gallon of oil an hour (or its equivalent in other fuels) should be figured for each 75 to 100 bushels of heavy seed in the bin. This will provide reasonably efficient operation at a drying air temperature of 110 . To get efficient operation in drying lighter seeds, proportionately larger batches are needed. One gallon an hour, for example, should be allowed for each 125-150 bushels of oats in drying at 110 .

The smallest driers on the market usually are rated at 3 horsepower. Generally it is wise to have a drier larger than one estimates for several reasons. Drying small seeds requires considerably more power than large seeds for the same depth. Drying at the minimum recommended rate of airflow is slow; within limits, higher rates reduce the time of drying appreciably. The higher rates also result in a more uniform moisture content. At the minimum specified rate, a big difference between driest and wettest layers will occur, and much of the seed will be dried more than is necessary for safekeeping. The producer will lose money in selling the overdried seeds because of the extra loss in weight.

The drier should be of a size that will let it keep up with the rate of harvesting. Rather than install too large a drier, however, a farmer may well start harvesting a few days earlier, if possible, and so spread harvesting over a longer period.